Surface treated steel product prepared by tin-based plating or aluminum-based plating

ABSTRACT

A tin-plating or aluminum-plating surface treated steel material with excellent corrosion resistance containing an intermetallic compound composed of one or more Group IIa (alkaline earth metal) elements and one or more Group IVb elements in a tin-based plating layer or aluminum-based plating layer on the surface of a steel material. For a tin-based plating layer, the long diameter of the intermetallic compound massive bodies is 1 82 m or greater and the short diameter/long diameter ratio is at least 0.4. For an aluminum-based plating layer, the long diameter of the intermetallic compound massive bodies is 10 μm or greater and the short diameter/long diameter ratio is at least 0.4.

TECHNICAL FIELD

The present invention relates to a surface treated steel material withexcellent corrosion resistance, to be used for outer plates, exhaustsystem members and gasoline tank materials of automobiles, metalconstruction materials for roofs and walls, public works materials, andhome and industrial electrical appliances.

BACKGROUND ART

Zn, Zn—Al, Al—Si, Sn, Pb—Sn, Cr and Ni platings are among platings usedfor surface treated steel materials, and because of their excellentcorrosion and heat resistance and attractive outer appearance, they havecome into wide use for automobile members, construction materials,electrical appliances and container materials. The most commonly usedplatings are Zn and Zn—Al platings. This is because Zn is the onlyplating metal with a sacrificing protection property to preventcorrosion of exposed iron when the basis iron is exposed. However, therelatively high corrosion rate of the Zn plating itself is a problem.With a Zn—Al system, the plating corrosion rate is lower but thesacrificing anti-corrosion action for iron is weaker. Elements thatimprove the corrosion resistance of the Zn plating itself generally tendto impair the sacrificing anti-corrosion effect, and therefore thecorrosion resistance of the plating itself and the anti-corrosion foriron facings are in an antimonious relationship.

Sn platings, Al—Si platings and the like that exhibit excellentcorrosion resistance as platings themselves all lack protective actionon basis iron exposed in normal environments.

The following is an example of such a plating for protection of exposedbasis iron. Specifically, U.S. Pat. No. 3,026,606 describes hot-dipaluminum plating of iron articles in an aluminum bath containing Mg andSi in a stoichiometric relationship in order to obtain Mg₂Si up to 25%in the plating layer. According to this patent, it is indeed possible toachieve improvement in corrosion resistance by crystallization of Mg₂Siin the aluminum plating layer. However, when Mg is added to the platingbath in excess of 10%, oxidation of the Mg causes highly profuseproduction of an oxide film on the plating bath, thus making continuousproduction impossible. Based on investigation by the present inventors,crystallization of Mg₂Si in an aluminum plating layer has been found toalter the form from fine to coarse, and thus notably influence thecorrosion resistance.

Japanese Examined Patent Publication HEI No. 3-21627 discloses anAl—Zn—Si—Mg quaternary alloy plating containing dendritic crystalscomposed mainly of aluminum and zinc. While it is indeed possible toadequately protect exposed iron with a crystallized plating of dendriticcrystals composed mainly of aluminum and zinc, the Zn is added in anamount of 25% or greater, which impairs the corrosion resistance of theplating itself.

It is an object of the present invention to provide plated steel sheetsexhibiting both high corrosion resistance of the plating itself and aprotective effect on exposed basis iron in a continuous productionprocess, which had been considered impossible to simultaneously achieve

DISCLOSURE OF THE INVENTION

According to the present invention there has been successfully achieveda surface treated steel material based on a completely different conceptunlike the conventional concept of protection of basis iron bysacrificing anti-corrosion action and plating base metal corrosionproduct coating action.

It is known in the prior art that corrosion resistance is improved bythe coating action of stabilized corrosion products obtained by additionof Mg to Zn-based platings. As a result of repeated diligent study onways of exhibiting the corrosion inhibitor effect of Mg even in Al-basedand Si-based platings as well as Zn-based platings, the presentinventors have completed the present invention upon finding that byincluding Mg in the plating as a water-soluble intermetallic compound(Mg₂Sn or Mg₂Si) in the form of massive bodies of consistent sizes, theintermetallic compounds elute out from the plating coat by contact withwater in a corroding environment and form an anti-corrosion filmcomposed mainly of Mg hydroxide, thus allowing a notable improvement inthe plating corrosion resistance.

As a result of searching for elements with anti-corrosion action inaddition to Mg, it was found that elements of the same Group as Mg,Group IIa of the Periodic Table (alkaline earth metals), exhibit ananti-corrosion corrosion effect. The anti-corrosion effect is most withMg and Ca among the alkaline earth metals.

Intermetallic compounds are generally considered to be poorly soluble inwater, but combinations of elements with a large difference in potentialnegativity are water-soluble. Much research has been conducted onelectronegativities of elements, but here we will rely on the valuesfrom the research by Pauling. Intermetallic compounds composed ofelements having a minimum/maximum electronegativity ratio of 0.73 orlower are soluble in water. The electronegativities of alkaline earthmetals are generally low and intermetallic compounds containing theseelements are readily soluble in water, but upon examining solubilitiesin water, it was found that intermetallic compounds composed of alkalineearth metals and Group IVb elements have considerably high solubilitiesin water. The most preferred Group IVb elements that form intermetalliccompounds with Mg and Ca are Si and Sn, because of theirelectronegativities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a tin-plating oraluminum-plating surface treated steel material with excellent corrosionresistance according to the invention, where the surface of the basisiron 1 has a tin-plating or aluminum-plating layer via a layer 2 of analloy between the basis iron and the plating metal, and there aredispersed in the plating layer 3 massive bodies 4 of an intermetalliccompound composed of a Group IIa (alkaline earth metal) and Group IVbelement.

FIG. 2 is an illustration showing a 5° inclined cross-sectionalstructure of a Sn-1% Mg-0.005% Ca-plated steel sheet.

FIG. 3 is an illustration showing a 50 inclined cross-sectionalstructure of an Al-8% Si-6% Mg-plated steel sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be explained in further detail.

In most hot-dip platings, an alloy layer is produced which is anintermetallic compound layer comprising Fe and the plating metal,referred to as an alloy layer, at the interface between the platinglayer and the basis iron. The “intermetallic compound” of the inventionis different and refers to an intermetallic compound present in theplating layer. Also, the “plating layer” mentioned here (throughout thespecification and in the claims) refers to the layer that does notinclude the layer produced at the interface between the plating layerand the basis iron, and a clear distinction is made between the platinglayer and the alloy layer.

According to the invention, sufficient corrosion resistance may beachieved by the presence of an intermetallic compound comprising a GroupIIa and Group IVb element in the plating layer, in the case of an Snplating layer, and the intermetallic compound is preferably present inthe plating layer in the form of massive bodies that are localized atspecific sites In the case of an Al plating layer, the intermetalliccompound comprising the Group IIa and Group IVb elements must be presentin the form of massive bodies.

According to the invention, the intermetallic compound comprising theGroup IIa and Group IVb elements dissolve and elute out in water incorrosive environments, forming an anticorrosion film on the platinglayer or basis iron. Formation of this anticorrosion film requiresdissolution of a fixed amount of the intermetallic compound in thecorrosive environment. With plating layers in which the intermetalliccompound is finely diffused, the metal itself composing the platinglayer must also corrode to some extent until the fixed amount of theintermetallic compound dissolves. Formation of the anticorrosion film bythe initial corrosion is therefore difficult to accomplish. Particularlywhen the plating is of Al or Sn which themselves are metals withexcellent corrosion resistance, the anticorrosion film forms later, andfurthermore, since the Al and Sn themselves exhibit no sacrificinganticorrosion function, the anticorrosion effect against the basis ironis notably lacking. In contrast, when the intermetallic compound isdiffused in the plating layer in the form of massive bodies, theintermetallic compound can be present in a sufficient amount to form ananticorrosion film even near the surface of the plating, and therefore,the Mg or Ca with the anticorrosion action are adequately released intothe environment upon the initial corrosion, thus allowing formation ofan anticorrosion film on the plating layer and basis iron. Particularlyin the case of an Al plating, the Group IIa element supplied from theintermetallic compound tends to adsorb onto the Al plating surface, sothat a Group IIa element-based anti-rust film is formed on the platingsurface as well. Consequently, in order to guarantee an amount of theGroup IIa element required for formation of an anti-rust film on thebasis iron, more of the Group IIa element (as the intermetalliccompound) is necessary than in the case of an Sn plating with lowadsorption on the plating surface. With an Al plating, therefore, it isessential for the intermetallic compound comprising a Group IIa andGroup IVb element to be present in the plating layer in the form ofmassive bodies.

Furthermore, since the intermetallic compound is usually harder than theplating layer, working will produce cracks in the plating layer startingparticularly from the intermetallic compound massive bodies, and sincedissolution of the intermetallic compound will commence from thosepoints, the presence of the intermetallic compound in the plating layerin the form of massive bodies will also give vastly superior corrosionresistance to the worked sections.

Moreover, the elements forming the intermetallic compound are at leastone element of Group IIa (alkaline earth metals) and at least oneelement of Group IVb. As mentioned above, this is because the solubilityof the intermetallic compound in water will thereby be notablyincreased. Preferred alkaline earth metals are Mg and Ca, because oftheir notable corrosion inhibitor effects on metals. Si and Sn areexamples of Group IVb elements that form water-soluble intermetalliccompounds with these alkaline earth metals. Compounds produced by theeelements are especially recommended. More preferred are Mg₂Si and Mg₂Sn.Intermetallic compounds comprising Group IIa and Group IVb elements arenot limited to binary systems, but also naturally include ternary andgreater systems.

The present invention is characterized by having a plating layer inwhich there is diffused an intermetallic compound with a large corrosioninhibitor effect, wherein at least a portion of the intermetalliccompound is in the form of massive bodies. The “massive bodies” arerelatively coarse with a small difference between long diameter andshort diameter, and the structure thereof is confirmed by inclinedcross-section. According to the invention, a “massive crystal” isdefined as one where the long diameter of the intermetallic compound isat least 1 μm for an Sn-based plating and at least 10 μm for an Al-basedplating, and a ratio of the short diameter to long diameter is at least0.4, as observed by a 50 inclined cross-section (polishing at an angleof 5° with respect to the surface of the steel sheet). Here, the shortdiameter and long diameter are the longest dimension (diameter) andshortest dimension (diameter) of a given crystal. Polishing alone, withno etching, is used for the observation. These intermetallic compoundsare water-soluble and also dissolve very easily in etching solutions.

The intermetallic compound may be identified, for example, by X-raydiffraction or EPMA analysis, but there is no limitation to these. Thecross-sectional structure may be observed with an optical microscope, aSEM, or the like, to examine the intermetallic compound on a structurallevel. The structure of the intermetallic compound is determined by thecharacteristic X-ray image by EPMA, or by quantitative analysis.Approximately 5° inclined polishing is preferred for the structureobservation, as this allows easy observation of the structure with anoptical microscope. EPMA analysis may be carried out with verticalpolishing and inclined polishing, but the analysis must be made withoutetching. The major components of the intermetallic compound may beidentified by X-ray diffraction. However, when the amount of theintermetallic compound is low compared to the plating layer, EPMA andstructural observation must be carried out together since the detectionsensitivity is too low with X-ray diffraction. Intermetallic compoundssuch as Mg₂Si can also be identified by structure using an opticalmicroscope. For example, the etching properties of different metals andintermetallic compounds containing Mg₂Si in various corroding solutionshave been elucidated, as described in “Aluminum Structure andProperties” (Keikinzoku Gakkai, ed., 1991), p.15, Table 4, and thereforevarious etching solutions can be used to identify Mg₂Si based onstructural observation.

Also, in order to obtain Mg₂Si or Mg₂Sn and Ca₂Si or CaSi in the form ofmassive bodies necessary to achieve stable corrosion resistance, thecooling rate of the plated steel sheet must be controlled in the stripcoding section. With conventional Al-Si based platings, the cooling ratein the strip cooling section must be at least 20° C./sec for refinementof the needle-like Si crystals in the plating layer that are a cause ofreduced corrosion resistance and workability. In the plating of theinvention, with a cooling rate of 20° C./sec or higher the Mg₂Si orMg₂Sn and Ca₂Si or CaSi massive bodies become fine and corrosionresistance cannot be adequately exhibited from the edges or facings. Thecooling conditions are therefore preferably below 20° C./sec, and morepreferably 3-15° C./sec. In particular, the Mg₂Si massive bodiescrystallize as the initial crystals upon aggregation of the hot-dipplating components, and it is important to slowly cool from the Mg₂Sicrystallization temperature (which differs depending on the plating bathcomposition) to near the eutectic temperature. The amount ofcrystallization for an aluminum-based plating is preferably from 5 to 40Mg₂Si massive bodies with a long diameter of 10 μm or greater, in afield of 1 mm plating width with a 5° inclination cross-section. For atin-based plating, it is preferably from 3 to 50 Mg₂Sn and Mg₂Si massivebodies with a long diameter of 1 μm or greater, in a field of 1 mmplating width with a 5° inclination cross-section. If the amount ofcrystallization is too low it will not contribute significantly tocorrosion resistance, and if it is too high it may adversely affect theworkability, while those sections may dissolve tending to result in aplating layer with abundant defects.

The main plating metal types according to the invention are thosecomposed of Al and Sn. The invention gives a protective effect on basisiron to Al and Sn platings that have conventionally been thought to lacka protective effect on basis iron, despite the excellent corrosionresistance of the platings themselves. In addition, for usesparticularly requiring edge anti-rust properties over long periods, itis preferred to select plating types containing small amounts of Zn. Theplating method of the invention is not particularly restricted, andhot-dip plating, vacuum deposition and the like may be employed.However, hot-dip plating is most preferred considering that the presentinvention will achieve active use of the intermetallic compound andcrystallize the intermetallic compound by aggregation of the hot-dipcomponents.

The plating layer components of the invention will now be explained.Here, the concentrations of each of the elements include the platinglayer and the intermetallic compound diffused throughout the platinglayer.

When Sn is selected as the main plating metal, the plating layercomponents are included, in terms of weight percentage, with either orboth Mg and Ca in ranges of Mg: 0.2-10%, Ca: 0.01-10%, optionally withAl at 0.01-10%, with the remainder Sn and unavoidable impurities, whilehaving an intermetallic compound composed of a Group IIa element andGroup IVb element in the plating layer. It is also effective to add Znat 1-40% and/or Si at 0.1-0.5%. The Group IIa elements Mg and Ca formthe intermetallic compounds Mg₂Sn and Ca₂Sn with the Group IVb elementSn, which contribute to corrosion resistance. The effect of improvedcorrosion resistance is active with at least 0.2% of both Mg and Ca,while at greater than 10% the melting point increases and a Mg oxidefilm is rapidly produced, thus impairing the manageability. Since Mg₂Snreadily takes the form of a diffused compound, there are no particularrestrictions on the form of the intermetallic compound in the Sn-basedplating, but, as observed with a 5° inclination cross-section the longdiameter of the intermetallic compound is preferably at least 1 μm andthe proportion of the short diameter to the long diameter is preferablyat least 0.4, as observed with a 5° inclination cross-section. Morepreferably, the long diameter of the intermetallic compound is at least3 μm and the proportion of the short diameter to the long diameter is atleast 0.4, Addition of Al and Ca inhibits oxidation of Mg and iseffective for obtaining a satisfactory outer appearance, for whichpurpose it is effective to add Al in an amount of at least 0.01% andpreferably at least 0.2%, and Ca in an amount of at least 0.01% and morepreferably at least 0.2%, considering the desired improvement incorrosion resistance; however, at over 10% the melting point rises, thusimpairing the manageability. When Zn is further added to Sn the Znprovides a sacrificing anticorrosion effect, which effect is exhibitedwith addition of at least 1%, and because an amount of greater than 40%increases the solubility of the plating layer, the upper limit ispreferably 40%, and more preferably 20%. Addition of Si results inproduction of Mg₂Si and Ca₂Si and improves the corrosion resistance, andit is therefore added to at least 0.1%. At greater than 0.5% the meltingpoint rises, thus impairing the manageability.

When Al is selected as the main plating metal, the plating layercontains intermetallic compound massive bodies composed of a Group Iaelement and Group IVb element. It is preferred for the long diameter ofthe intermetallic compound massive bodies to be at least 10 μm and theproportion of the short diameter to the long diameter to be at least0.4, in order to achieve stable corrosion resistance. More preferably,the long diameter of the intermetallic compound is at least 15 μm andthe proportion of the short diameter to the long diameter is at least0.4. The plating layer components are preferably included, in terms ofweight percentage, with either or both Mg and Ca in ranges of Mg: 2-10%,Ca: 0.01-10%, and with Si: 3-15%, with the remainder Al and unavoidableimpurities. Si is known as an element that inhibits growth of metalplating layers, and it exhibits its effect when added to at least 3%,and preferably over 6%. However, if added in large excess it raises themelting point of the plating bath resulting in excessive growth of thealloy layer, which has been implicated as a cause of reducedworkability, and therefore the upper limit for Si is 15%.

Mg improves the corrosion resistance when added at 2% or greater, and itis preferably added at 4% or greater. The present invention forms Mg₂Simassive bodies in the aluminum plating layer, and the Mg/Si ratio of theplating layer is preferably slightly lower than 1.73 equivalents ofMg₂Si. When the Mg/Si ratio is in the range of 1.70 or lower, theplating layer becomes a ternary eutectic composition of Al—Mg₂Si—Si,which provides the maximum corrosion resistance. This is conjectured toresult from the fact that the melting point is lowest within this range,such that the alloy layer growth is inhibited and the amount of theplating layer contributing to corrosion resistance is substantiallyincreased. However, excessive addition raises the melting point of theplating bath, resulting in excessive growth of the alloy layer, whichhas been implicated as a cause of reduced workability, and rapidproduction of an Mg oxide film; the upper limit for Mg is therefore 10%.

Ca is also preferably added to at least 0.01% in the plating layer. Thisis because Ca inhibits oxidation of Mg in the molten metal duringhot-dip plating, thus preventing defects in the outer appearance. Whenplating is carried out in air without addition of Ca, a severe wrinkledpattern is produced on the plating surface which lowers the productvalue, such that means for inhibiting the molten metal portions in a lowoxygen atmosphere is necessary, thus requiring investment for equipment.The Mg oxidation-inhibiting effect of the Ca addition is saturated at0.2%. When Ca is added in a greater amount it also reacts with Siforming Ca₂Si, CaSi and the like, providing an anticorrosion effectsimilar to that of Mg₂Si. When Ca is added, the (Ca+Mg)/Si mass ratio ispreferably no greater than 2.8 to cause crystallization of the Mg₂Si andCa₂Si, CaSi in the plating layer. However,.excessive addition of Caraises the melting point of the plating bath resulting in excessivegrowth of the alloy layer, which has been implicated as a cause ofreduced workability, and therefore the upper limit for Ca is 10%.

Addition of Zn also provides a sacrificing anticorrosion effect by theZn. This effect is exhibited with addition of 2% or greater, but atgreater than 25% the solubility of the plating layer increases andtherefore the upper limit is preferably 25%. More preferred is a lowerlimit of 10% and an upper limit of 20%.

In addition to Ca, Be is another element effective for inhibitingoxidation of Mg in any Al-based or Sn-based plating type, but because Beis a toxic element its application is highly undesirable.

The thickness of the plating layer is preferably 2-100 μm. For mostplating layers, increasing thickness is advantageous for corrosionresistance, and disadvantageous for workability and weldability. Thepreferred thickness for the plating layer will therefore differdepending on the use, and while a thinner plating layer is better forautomobile parts that require excellent workability and weldability,sufficient corrosion resistance cannot be guaranteed at less than 2 μm,and therefore the thickness is preferably at least 2 μm. On the otherhand, for construction materials and household appliance uses that donot demand particular workability or weldability, a larger plating layerthickness is better from the standpoint of improved corrosionresistance, but because of the drastically inferior workability atthicknesses exceeding 100 μm, it is preferably no greater than 100 μm.The present invention is also effective for frame and running automobileparts. Arc welding is usually employed for frame and running automobileparts, but in the case of Zn-based platings the high vapor pressure ofthe Zn has tended to produce blowholes. Al-based and Sn-based platingsthat have low vapor pressure would be normally preferred, but suchplatings have not come into use because of their weak protective effecton basis iron. According to the invention, even such highlycorrosion-resistant platings have a protective effect on basis iron, andoffer the additional advantage of producing no blowholes during arcwelding.

The roughness of the plating surface affects the outer appearance,corrosion resistance, weldability and workability. A high roughness isadvantageous for workability, but disadvantageous for weldability andcorrosion resistance. The optimum value will therefore differ dependingon the type of plating and its intended use, but it is preferably 3 μmor lower in terms of Ra.

All Al-based and Sn-based plated steel materials produce alloy layers atthe interface between the plating layer and the basis iron. Thethickness is about 0.1-1 μm in the case of Sn-based platings that have alow melting point, and reaches 0.5-5 μm in the case of Al-basedplatings. Because the thickness of the alloy layer has a particularlylarge influence on the workability and post-working corrosion resistancein the case of an Al-based plating, the thickness of the alloy layer ispreferably no greater than 5 μm.

For improved corrosion resistance, a thinner alloy layer and improvedplating wettability, it is preferred to form a pre-plating containingone or more from among Ni, Co, Zn, Sn, Fe and Cu at the interface withthe plating layer and the basis iron, as treatment prior to plating. TheAl or Sn is hot-dip plated after the pre-plating, or in the case of heattreatment, an alloy layer is formed between the pre-plating layer andthe basis iron, or between the pre-plating layer and the plating layer.The pre-plating layer and alloy layer may also be formed as a mixedlayer, and the gist of the invention is not impaired regardless of thestate. The pre-plating is sometimes dissolved in the plating bath, orthe pre-plating components are sometimes included in the plating layeror steel sheet by diffusion, and this will also not impair the gist ofthe invention.

The constituent elements of the plating are basically the main platingmetal and intermetallic compound-forming elements and unavoidableimpurities, but if necessary Bi, Sb, Fe, misch metal, Be, Cr, Mn or thelike may also be added.

By using a post-treatment coating such as a chemical treatment coatingor resin coating on the uppermost surface of the plating layer, aneffect of improved weldability, coating cohesion and corrosionresistance may be expected. As chemical treatment coatings there may beused chromic acid/silica-based coatings, silica/phosphoric acid-basedcoatings, silica/resin-based coatings and the like, and as resincoatings there may be used acrylic-based, melamine-based,polyethylene-based, polyester-based, fluorine-based, alkyd-based,silicon polyester-based and urethane-based resins as well as othercommon resins. There are no particular restrictions on the filmthickness, but treatment may usually be carried out to 0.2-20 μm.Inhibitors using no chrome are recently being investigated aspost-treatment, and such treatment may of course be used.

The steel components of the basis metal will now be explained. There areno particular restrictions on the steel components, and an improvingeffect on corrosion resistance is exhibited for any type of steel; thesteel type may be IF steel, Al-killed steel, Cr-containing steel,stainless steel, high-tension steel, etc. to which Ti, Nb, B or the likehas been added. Al-k steel or stainless steel is preferred forconstruction uses, Ti—IF steel is preferred for exhaust systems, Al-ksteel is preferred for household appliances, B-added IF steel ispreferred for fuel tanks, and electromagnetic steel sheets are preferredfor magnetoelectric shields.

EXAMPLES

The present invention will now be explained in greater detail by way ofthe following examples.

Example 1

Hot-dip tin plating was performed using as the material a cold-rolledsteel material (sheet thickness: 0.8 mm) with the steel components shownin Table 1, which had been subjected to normal hot rolling and coldrolling steps.

First, a Ni plating was formed to about 1 g/m² by an electroplatingmethod using a Watt bath. A flux method was then used for tin plating.After plating, the plating coverage was adjusted by gas wiping. Theplated steel sheet was then cooled and rolled up.

Plating was then carried out with appropriate variations in the amountsof Mg, Ca and Al in the plating bath composition. Also included asunavoidable impurities from the plating instruments and stripping in thebath were Fe and Ni, at no greater than 0.05% each in the plating bath.The bath temperature was 260-300° C.

The outer appearance of the platings were satisfactory with nonon-plated sections, but severe oxidation on the bath surface wasobserved depending on the bath composition. The plating coverage wasuniform on both sides, at about 60 g/m² on both sides, and the surfaceroughness was 0.9-1.4 μm in terms of Ra.

FIG. 2 shows a photograph (200×) of the 50° inclined cross-sectionalstructure of the plating layer of a plated sample using a Sn-1% Mg-0.01%Ca bath. The Mg₂Sn granular phase is shown diffused in the plating, andthe presence of this compound was also confirmed by X-ray diffraction.In the photograph of FIG. 2, the lower gray portion is the cross-sectionof the basis iron, and the upper portion with the thick linear patternis the surface of the plating layer (plan view); the white (light gray)portion at the intermediate region is the cross-section (50° inclinedcross-section) of the plating layer. The granular intermetallic compound(Mg₂Sn) is seen present as groups of black lined dots in the 50°inclination cross-section of the white plating layer.

A pure Sn-plated steel sheet and a Pb-8% Sn-plated steel sheet were alsomanufactured for comparison. Both were plated after Ni pre-plating. Theplated steel sheets contained no intermetallic compounds in the platinglayers. Their performance was evaluated by the tests described below.

TABLE 1 Steel components of test materials (wt %) C Si Mn P S Ti Al N NbB 0.0012 0.02 0.22 0.007 0.010 0.05 0.03 0.002 0.004 0.0005

(1) Plating Layer Analysis

[1] Plating Layer Composition Analysis Method

Both sides of a 50×50 sample were subjected to electrostripping in a 5%NaOH solution (wt %) at a current density of 10 mA/cm², using stainlesssteel as the counter electrode. Upon abrupt increase in the potential,the current density was gradually reduced to half, and then eventuallyreduced to 1 mA/cm², and electrolysis was suspended when the potentialof the Ni layer or alloy layer was exhibited The scrap adhering to thesteel sheet was gently wiped with absorbent cotton, and the analysissolution was sampled together with it.

The analysis solution was then filtered and the undissolved scrap wasdissolved in 10% hydrochloric acid. The filtrate and solution werecombined and subjected to quantitative analysis by ICP (inductivelycoupled plasma) emission spectroscopy.

Because error due to Cr, Si, etc. can occur when a is steel sheet issubjected to chemical treatment, the stripping may be carried out afterlight paper polishing of the surface.

[2] Plating Composition Observation Method

The plating layer cross-section was polished at a 5° inclination, andthe plating composition was observed (200-500×) with an opticalmicroscope. The long diameter and number of the intermetallic compounds(long diameter/short diameter ratio ≧0.4) in the plating layer weremeasured within a 1 mm width (arbitrary) field of the plating.

(2) Corrosion Resistance

[1] Salt Injury Corrosion Resistance

After cross-cutting a 70×150 mm sample, a salt spray test was carriedout according to JIS Z2371 and the time until generation of red rust wasevaluated.

Evaluation Scale

◯: red rust generated after 20 days

Δ: red rust generated at 10-20 days

X: red rust generated within 10 days

[2] Post-painting Corrosion Resistance

A 70×150 mm sample was subjected to chromic acid/silica chemicaltreatment to about 20 mg/m² in terms of metal Cr, and then to paintingwith melamine-based black paint to 20 μm, followed by 20 minutes ofbaking at 140° C. This was then cross-cut and provided for a salt spraytest. The outer appearance after 60 days was visually evaluated.

Evaluation Scale

⊚: no red rust generation

◯: no red rust generation other than from cross-cut

Δ: red rust generation of ≦5%

X: red rust generation of >5%

[3] Fuel Corrosion Resistance

The corrosion resistance against gasoline was evaluated. The methodinvolved placing a test solution in a sample subjected to deep drawingwith a flat-bottomed cylinder having a 20 mm flange width, 50 mmdiameter and 25 mm depth with a hydraulic molding tester, and capping itwith glass via a silicon rubber ring. The state of corrosion after thetest was visually observed.

Test Conditions

Test solution: gasoline+10% distilled water+200 ppm formic acid

Test period: standing for 3 months at 40° C.

Evaluation Scale

◯: red rust generation of <0.1%

Δ: red rust generation of 0.1-5% or white rust

X: red rust generation of >5% or considerable white rust

[4] Outdoor Exposure Test

Coating was carried out after chemical treatment. The coating was withtwo types of epoxy-based resins (20 μm). The sample was sheared todimensions of 50×200 nm and subjected to an outdoor exposure test. Afterthe course of one month, the generation of red rust from the edges andthe condition of discoloration of the surface were evaluated.

Evaluation Scale

◯: red rust generation of >30% from edges

Δ: red rust generation of 30-80% from edges

X: red rust generation of >80% from edges

(3) weldability

Spot welding was performed under the welding conditions indicated below,and the number of continuous weld points up to the time at which thenugget system cleared 4{square root over (t)} (t=sheet thickness) wasevaluated.

Welding Conditions

welding current: 10 kA

Pressure force: 220 kg

Welding time: 12 cycles

Electrode diameter: 6 mm

Electrode shape; dome, 6Φ-40R tip

Evaluation Scale

◯: more than 1000 continuous weld points

Δ: 500-1000 continuous weld points

X: fewer than 500 continuous weld points

(4) Workability

Cup molding was carried out with a hydraulic molding tester using a50-mm diameter cylindrical punch at a draft of 2.25. The test wascarried out with oiling, and the wrinkle holding pressure was 500 kg.The workability was evaluated according to the following index.

Evaluation Scale

◯: no anomalies

Δ: cracks in plating

X: peeling of plating

TABLE 2 Summary of plating levels Intermetallic compound Produc- massivetion bodies (in 1 mm) condi- width field of tions plating layer Coolingcross-section) temper- Mean Plating layer ature long di- composition (°C./ ameter No. Mg Ca Al Zn Si sec) (μm) Number Remarks 1 0.5 — 0.5 — —16 3  3 Invention Example 2 1 — 0.5 — — 16 4 12 Invention Example 3 2 —0.5 — — 16 4 13 Invention Example 4 3 — 1   — — 16 5 13 InventionExample 5 5 — 3   — — 16 5 16 Invention Example 6 1 0.05 — — — 16 3 12Invention Example 7 2 0.1  — — — 16 4 14 Invention Example 8 2 0.03 0.2— — 16 4 14 Invention Example 9 2 0.03 0.2 — — 30   0.5 13 InventionExample 10 — 1   1   — — 16 3 12 Invention Example 11 1 — 0.5 8 — 16 212 Invention Example 12 1 0.05 — — 0.2 16 3 13 Invention Example 13 Sn16 — — Comp. Ex. 14 Sn-8% Zn 16 — — Comp. Ex. 15 Pb-8% Sn 16 — — Comp.Ex.

TABLE 3 Performance measurement results Corrosion resistance OverallSalt After Weld- Work- evalua- No. injury coating Fuel Exposure abilityability tion Remarks 1 ∘ ∘ ∘ ∘ ∘ ∘ ∘ Invention Example 2 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚Invention Example 3 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚ Invention Example 4 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚Invention Example 5 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚ Invention Example 6 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚Invention Example 7 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚ Invention Example 8 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚Invention Example 9 ∘ ∘ ∘ ∘ ∘ ∘ ∘ Invention Example 10 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚Invention Example 11 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚ Invention Example 12 ∘ ⊚ ∘ ∘ ∘ ∘ ⊚Invention Example 13 x ∘ Δ x ∘ ∘ x Comp. Ex. 14 Δ ∘ ∘ x ∘ ∘ x Comp. Ex.15 x ∘ Δ x ∘ ∘ x Comp. Ex. Overall evaluation ⊚ very excellent, ∘excellent, Δ somewhat inferior but usable, x unusable

The Pb-8% Sn-plated steel sheet listed as No.15 which is widely used forconventional automobile fuel tanks and the Sn-plated steel sheet listedas No.13 had excellent corrosion resistance of the platings themselves,but there was no protective effect on the basis iron exposed at theedges or at the unplated sections. The Sn-8% Zn-plated steel sheet ofNo.14 showed improvement in this aspect, but it was insufficient.

In contrast, the invention examples of Nos. 1-12 had very excellentcorrosion resistance. However, because No.1 had a low Mg content, andNo.9 had a high cooling rate at the melting pot exit end and a smallintermetallic compound grain size, their effects were insufficient.

Production of Mg₂Sn and Ca₂Sn was confirmed in all of the examples byX-ray diffraction and cross-sectional inclined polishing, and theexcellent corrosion resistance of the invention examples was attributedto the passivation effect on the plating layer and basis iron due todissolution of these water-soluble intermetallic compounds.

Example 2

Hot-dip aluminum plating was carried out using as materials cold-rolledsteel sheets with the same steel components and sheet thicknesses as inExample 1.

The hot-dip aluminum plating was accomplished using a non-oxidizingfurnace-reducing furnace type line, and the plating coverage wasadjusted by gas wiping after plating, after which the sheet was cooledand subjected to zero spangle treatment. The composition of the platingbath was varied for production of different samples, and the propertieswere examined. Incidentally, the plating bath contained about 1-2% of Feas an unavoidable impurity from the plating instruments and stripping.The bath temperature was 640-660° C. There was no particularly severeoxidation of Mg or Ca. Under some conditions (no addition of Ca, or noN₂ seal BOX), however, wrinkles were observed on the outer appearance.By modifying the sheet penetration temperature and the cooling rateafter plating, it was possible to achieve a smaller alloy layerthickness, and the thickness achieved was 1.5-3 μm.

The plating coverage was approximately 60 g/m², uniformly on both sides.The surface roughness was 1.2-2.2 μm in terms of Ra.

A 5° inclination cross-sectional structure with a plating layercomposition of Al-8%Si-6%Mg-0.l%Ca is shown in FIG. 3. In the photographof FIG. 3 (200×), the lower gray portion is the cross-section of thebasis iron, the nearly white portion at the middle region is thecross-section of the plating layer (5° inclined cross-section), theunfocused top area is the surface of the plating layer, and at theinterface between the basis iron and the plating layer there can be seena thin alloy layer, though it is difficult to distinguish since itscolor is similar to that of the basis iron in this photograph. The Mg₂Simassive bodies are seen as relatively dark gray triangular to hexagonalshapes in the white plating layer cross-section.

The short axes of the Mg₂Si massive bodies of the sample produced herewere 4-25 μm, the long axes were 6-30 μm, and the short diameter/longdiameter ratios were 0.7-1. The Mg₂Si was also present in a finegranular phase in addition to this massive structure. X-ray diffractionand EPMA analysis also confirmed the presence of Mg₂Si. The added Mg hadbeen almost totally converted to Mg2Si, and was estimated to be presentat about 9% in this plating layer composition.

For comparison, a conventional type of aluminum plating, i.e. Al-10% Siplating and a Galvalume steel sheet (Zn-55% Al-1.5% Si) were alsoproduced. The coverage for these was 60 g/m2 on both sides.

(1) Plating Layer Analysis Method

[1] Plating Layer Composition Analysis Method

Both sides of a 50×50 sample were subjected to electrostripping in a 3%NaOH+1% AlCl₃.6H₂O solution (wt %) at a current density of 20 mA/cm²,using stainless steel as the counter electrode. Upon abrupt increase inthe potential, the current density was gradually reduced to half, andthen eventually reduced to 1 mA/cm², and electrolysis was suspended whenthe potential of the alloy layer was exhibited. Black scrap was producedbecause Mg₂Si, Ca₂Si, etc. are insoluble in such alkali solutions.Electrostripping was then repeated with 5% NaCl. The current densityhere was initially 10 mA/cm² and upon abrupt increase in the potential,the current density was gradually reduced to half and then to 1 MA/cm².The insoluble scrap was gently wiped from the steel sheet with absorbentcotton, and each absorbent cotton was used as a sample analysissolution. The analysis solution was then filtered and the undissolvedscrap was dissolved in 10% hydrochloric acid. The filtrate and solutionwere combined and subjected to quantitative analysis by ICP (inductivelycoupled plasma) emission spectroscopy. Because error due to Cr, Si, etc.can occur when a steel sheet is subjected to chemical treatment, thestripping may be carried out after light paper polishing of the surface.

[2] Plating Composition Observation Method

The plating layer cross-section was polished at a 5° inclination, andthe plating composition was observed (200-500×) with an opticalmicroscope. The long diameter and number of the intermetallic compounds(Mg₂Si massive bodies with long diameter/short diameter ratio ≧0.4) inthe plating layer were measured within a 1 mm width (arbitrary) field ofthe plating.

(2) Corrosion Resistance Evaluation

[1] Salt Injury Corrosion Resistance

A salt spray test according to JIS Z2371 was carried out for 30 day on a70×150 mm sample, and the corrosion product was stripped for measurementof the corrosion loss. The corrosion loss was expressed as a value withrespect to one plated side.

Evaluation Scale

⊚: corrosion loss of ≦5 g/m²

◯: corrosion loss of <10 g/m²

Δ: corrosion loss of 10-25 g/m²

X: corrosion loss of >25 g/m²

[2] Post-painting Corrosion Resistance

Chromic acid/silica chemical treatment was carried out as chemicaltreatment to about 20 mg/m² in terms of metal Cr. This was followed bypainting of a 70×150 mm sample with melamine-based black paint to 20 μm,and then 20 minutes of baking at 140° C. This was then cross-cut andprovided for a salt spray test. The outer appearance after 60 days wasvisually evaluated.

Evaluation Scale

⊚: no red rust generation

◯: no red rust generation other than from cross-cut

Δ: red rust generation of ≦5%

X: red rust generation of >5%

[3] Fuel Corrosion Resistance

The corrosion resistance against gasoline was evaluated. The methodinvolved placing a test solution in a sample subjected to deep drawingwith a flat-bottomed cylinder having a 20 mm flange width, 50 mmdiameter and 25 mm depth with a hydraulic molding tester, and capping itwith glass via a silicon rubber ring. The state of corrosion after thetest was visually observed.

Test Conditions

Test solution: gasoline+10% distilled water+200 ppm formic acid

Test period: standing for 3 months at 40° C.

Evaluation Scale

◯: red rust generation of <0.1%

Δ: red rust generation of 0.1-5% or white rust

X: red rust generation of >5% or considerable white rust

[4] Corrosion Resistance Against Exhaust System Condensation

A 25×100 mm sample was used in a test according to JASOM611-92B of theJapan Automobile Technical Specifications. The test period was 4 cycles.After the test, the corrosion product was stripped and the corrosiondepth was measured.

Evaluation Scale

◯: corrosion depth of <0.05 mm

Δ: corrosion depth of 0.05-0.2 mm

X: corrosion depth of >0.2 mm

[5] Outdoor Exposure Test

Coating was carried out after the chemical treatment described above in[2]. The coating was with two types, a polyethylene wax-containingacrylic resin (clear: 5 μm) and an epoxy resin (20 μm). The sample wassheared to dimensions of 50×200 nm and subjected to an outdoor exposuretest. After the course of 3 months, the generation of red rust from theedges and the condition of discoloration of the surface were evaluated.

Evaluation Scale

◯: red rust generation of <30% from edges

Δ: red rust generation of 30-80% from edges

X: red rust generation of >80% from edges

(3) Weldability

After the chemical treatment described above in [2], spot welding wasperformed under the welding conditions indicated below, and the numberof continuous weld points up to the time at which the nugget systemcleared 4{square root over (t)} (t=sheet thickness) was evaluated.

Welding Conditions

Welding current: 10 kA

Pressure force: 220 kg

Welding time: 12 cycles

Electrode diameter: 6 mm

Electrode shape: dome, 6 Φ-40R tip

Evaluation Scale

◯: more than 700 continuous weld points

Δ: 400-700 continuous weld points

X: fewer than 400 continuous weld points

(4) Workability

cup molding was carried out with a hydraulic molding tester using a50-mm diameter cylindrical punch at a draft of 2.25. The test wascarried out with oiling, and the wrinkle holding pressure was 500 kg.The workability was evaluated according to the following index.

Evaluation Scale

◯: no anomalies

Δ: cracks in plating

X: peeling of plating

(5) Outer Appearance

The outer appearance was visually observed after plating.

Evaluation Scale

◯: uniform outer appearance

Δ: thin wrinkled pattern

X: wrinkled pattern

TABLE 4 Summary of plating levels Intermetallic compound massive bodiesProduc- (in 1 mm width) tion field of condi- plating layer tionscross-section) Cooling Mean Plating layer tempera- long Outercomposition ture diameter appear- No. Si Mg Ca Zn (° C./sec) (μm) Numberance Remarks 1 4 6 0.1 — 15 20 20 good Invention Example 2 8 6 0.1 — 1520 25 good Invention Example 3 8 6 — — 15 23 23 wrinkled Inventionpattern Example 4 8 3 0.1 — 5 30 8 good Invention Example 5 8 6 0.1 5 1520 19 good Invention Example 6 8 6 0.1 12 15 20 20 good InventionExample 7 8 6 0.1 23 15 21 20 good Invention Example 8 10 6 5 — 15 24 2good Invention Example 9 12 8 0.5 — 15 26 25 good Invention Example 1014 — 10 — 15 25 16 good Invention Example 11 10 2 3 — 15 13 15 goodInvention Example 12 12 6 0.1 14 15 23 23 good Invention Example 13 8 60.1 35 6 2 good Comp. Ex. 14 9 6 0.1 30 7 10 good Comp. Ex. 15 10 6 0.115 35 6 6 good Comp. Ex. 16 Al-10Si 30 — — good Comp. Ex. 17Zn-55Al-1.5Si 25 — — good Comp. Ex.

TABLE 5 Performance evaluation results Corrosion resistance OuterOverall Salt After Exhaust Expo- Weld- Work- appear- evalua- No. injurycoating Fuel system sure ability ability ance tion Remarks 1 ∘ ∘ ∘ ∘ ∘ ∘∘ ∘ ∘ Invention 2 ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Example 3 ∘ ∘ ∘ ∘ ∘ ∘ ∘ x Δ 4 ∘ Δ ∘∘ Δ ∘ ∘ ∘ Δ 5 ∘ ⊚ ∘ ∘ ∘ ∘ ∘ ∘ ⊚ 6 ∘ ⊚ ∘ ∘ ∘ ∘ ∘ ∘ ⊚ 7 ∘ ⊚ ∘ ∘ ∘ ∘ ∘ ∘ ⊚8 ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 9 ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 10 ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ 11 ∘ Δ ∘ ∘Δ ∘ ∘ ∘ Δ 12 ∘ ⊚ ∘ ∘ ∘ ∘ ∘ ∘ ⊚ 13 ∘ x ∘ ∘ Δ ∘ ∘ ∘ x Comp. Ex. 14 ∘ x ∘ ∘Δ ∘ ∘ ∘ x 15 ∘ x ∘ ∘ Δ ∘ ∘ ∘ x 16 Δ x ∘ ∘ x ∘ ∘ ∘ x 17 x x Δ Δ Δ ∘ ∘ ∘ xOverall evaluation ⊚ very excellent, ∘ excellent, Δ somewhat inferiorbut usable, x unusable

Industrial Applicability

the present invention provides surface treated steel sheets with highcorrosion resistance of Sn-based platings and Al-based platingsthemselves which has not been achievable by the prior art, while alsoproviding an anti-corrosion effect on the edges and damaged sections.Because they can be used for virtually any type of conventional surfacetreated steel sheet, a very considerable industrial contribution isprovided.

What is claimed is:
 1. A tin-plating surface treated steel material withexcellent corrosion resistance, characterized by containing anintermetallic compound composed of one or more Group IIa (alkaline earthmetal) elements and one or more Group IVb elements in a tin-basedplating layer on the surface of a steel material.
 2. A tin-platingsurface treated steel material with excellent corrosion resistanceaccording to claim 1, wherein said intermetallic compound is a compoundcomposed of at least one element of Mg and Ca of Group IIa and at leastone element of Si and Sn of Group IVb.
 3. A tin-plating surface treatedsteel material with excellent corrosion resistance according to claim 1wherein said intermetallic compound is in the form of massive bodieswith a long diameter of 1 μm or greater and a short diameter/longdiameter ratio of 0.4 or greater.
 4. A tin-plating surface treated steelmaterial with excellent corrosion resistance according to claim 1,wherein the component composition of said tin-based plating layer is, interms of weight percentage, either or both Mg and Ca in ranges of Mg:0.2-10% and Ca: 0.01-10%, with the remainder Sn and unavoidableimpurities.
 5. A tin-plating surface treated steel material withexcellent corrosion resistance according to claim 1, wherein thecomponent composition of said tin-based plating layer is, in terms ofweight percentage, either or both Mg and Ca in ranges of Mg: 0.2-10% andCa: 0.01-10%, with Al in the range of 0.01-10%, and the remainder Sn andunavoidable impurities.
 6. A tin-plating surface treated steel materialwith excellent corrosion resistance according to claim 4, wherein saidtin-based plating layer further contains, in terms of weight percentage,Zn: 1-40% and/or Si: 0.1-0.5%.
 7. A tin-plating surface treated steelmaterial with excellent corrosion resistance according to claim 1,wherein the thickness of the plating layer on the steel material surfaceis 2-100 μm.
 8. A tin-plating surface treated steel material withexcellent corrosion resistance according to claim 1, wherein theroughness of the plating surface is no greater than 3 μm in terms of Ra.9. A tin-plating surface treated steel material with excellent corrosionresistance according to claim 1, wherein an alloy layer with a thicknessof no greater than 5 μm is present at the interface between the platinglayer and the steel material.
 10. A tin-plating surface treated steelmaterial with excellent corrosion resistance according to claim 1,having at least one of a pre-plating layer containing one or more fromamong Ni, Co, Zn, Sn, Fe and Cu, an alloy layer of said pre-platinglayer and the basis iron, and an alloy layer of said pre-plating layerand the plating layer.
 11. A tin-plating surface treated steel materialwith excellent corrosion resistance according to claim 1, containing oneor more added elements from among Ni, Co, Zn, Sn, Cu, Bi, Sb, mischmetal, Fe, Be, Cr and Mn, as a plating layer component.
 12. Atin-plating surface treated steel material with excellent corrosionresistance accordance to claim 1, having a post-treatment coating on theuppermost surface of the steel material.